This proposal responds to PA-01-072, "HIV-1 Infection of the Central Nervous System (CNS)." This study has two major goals. The first goal is to elucidate the mechanisms driving the trafficking of immune effector cells, particularly cytotoxic T-cells (CTL), to the CNS, and the effector functions of these cells that may either palliate or exacerbate neuropathogenesis. The second goal is to determine the relationship between trafficking of antiviral T-cells to cerebrospinal fluid (CSF) and viral load, antiretroviral therapy, and development of neurological disease. In preliminary studies, we determined that multiparameter flow cytometry may be used to detect antigen-specific CD8+ T-cells obtained "ex vivo" from 5-10 ml of CSF. In Specific Aim 1, we will study the frequency and trafficking patterns of HIV-specific CTL in CSF, and determine the relationship of CTL frequency in CSF to systemic and intrathecal immune activation, chemokine expression, HIV viral load and development of CNS disease. In Specific Aim 2, we will assess the effector functions of CSF CTL, including secretion of proinflammatory cytokines (IFN-gamma,, TNF-alpha), cytolytic effector molecules (i.e., perforin, granzymes), and MHC class I restricted lysis of target cells. We will also assess the clonality of HIV-specific CTL in CSF and peripheral blood, and determine the extent of"overlap" between these two populations. Two studies will be performed: first, a cross-sectional study of 25 HIV-infected patients with detectable HIV-specific CTL in perpheral blood; second, a longitudinal study of 40 HIV-infected patients including 10 who are stopping or interrupting highly-active antiretroviral therapy (HAART), 10 who are beginning HAART, 10 clinically stable individuals not on HAART and 10 with symptoms of neurological disease. We hypothesize that systemic lymphocyte activation (including upregulation of LFA-1 and VLA-4) leads to increased extravasation of activated lymphocytes into CSF, and that viral replication within the CNS, followed by a local inflammatory response and secretion of chemokines (i.e., IP-10, MIP-1alpha and MIP-1beta,RANTES) contributes to recruitment of cells expressing receptors for these chemokines in a non antigen-specific manner. We also hypothesize that there may be a correlation between intrathecal HIV viral load and T-cell recruitment to CSF, and that the presence of CTL in CSF may be predictive of neurological disease.